I'd say Tauren. Then orcs or draenei. I think worgen would have to be pretty high up there as well when transformed. Maybe on par with orcs and draenei. Dwarfs wouldn't be far behind either. They are a robust looking race.

In this case bigger does equal strongest. Tauren shamans carry giant totems with them everywhere they go. And these things are the size of trees. In WoW totems were made small due to gameplay. But in the RTS and the cinematics you can see them carrying the extremely heavy ones. Hell, Cairne Bloodhoof use to carry three!

Seeing as bovines are extremely strong in real life, and Tauren are anthropomorphic 3 meters tall cows/bulls/buffalos/wtfever they are probably physically strongest race in WoW.
Orcs,Worgen and Draenei could be on a pretty similar level of strength.